I'm not sure where to begin here. I can control the speed and incline but have no idea how to read/interpret the feedback. A nudge in the right direction is all I ask. Meanwhile, I'll keep searching the forum. All the info on the signals I have (full controller info attached):

Speed:This connection is for the reed switch wire. This allows the tach signal to be received by the controller and passed to the console wire harness, where it is sent up the GREEN wire. Voltage is a pulsing 5 VDC when the treadmill is running. When the treadmill is at rest, this voltage may be 0 VDC or 5 VDC, depending on the whether the magnet is closing the reed switch or not.

Incline:This connection is for the incline sensor. While it will usually be a reed switch, an optic switch can also be connected. 5 VDC will be present across the open switch. Whenever the switch closes, 0 VDC will be measured. As the incline motor turns, this voltage will pulse on and off. The number of pulses is used by the console to determine how far it has changed the incline. When the incline is at rest, this voltage may measure 0 VDC or 5 VDC, depending on the position of the switch in relation to the magnet (or optic disk).

This connection is for the reed switch wire. This allows the tach signal to be received by the controller and passed to the console wire harness, where it is sent up the GREEN wire. Voltage is a pulsing 5 VDC when the treadmill is running. When the treadmill is at rest, this voltage may be 0 VDC or 5 VDC, depending on the whether the magnet is closing the reed switch or not.

So, have you connected this wire to the Arduino? Are you seeing the pin go HIGH and LOW? Does the state switch often enough that polling is not sufficient? Have you tried using an interrupt?

The first part of the problem is connecting the sensor to the Arduino correctly. The second part is reading the sensor, at least some times. The third part is reading it every time. The fourth part is making sense of/using the data that you get. We have no clue where you are in this progression.

sending 5VDC to the mc2100 to trigger incline/decline through pins 10/11 down wires ORANGE/YELLOW

I can count the pulses and seem to be getting a fairly accurate reading where 5 pulses = 1 deg of incline. The only issue I have is figuring out the current incline as the sense signal only seems to cover define the current distance being moved. As of now, I think I just need to store the position or always zero out the incline on shutdown.

The second, reading the speed, is where I have some problems.

The GREEN speed sense from the controller connected to pin 3.

The BLUE wire connected to pin 9 and I'm using Timer 1 to set the PWM.

Here I can control the power but have no clue how to read the current speed.

I found out by reading the mc2100 datasheet, that the controller uses the green wire to send small digital signals to the control panel as well as the tach pulse that is sent up the green wire.

You can verify that by reading the signal by manually rotating the wheel past the sensor and reading its pulse. If you start the motor running, you can completely remove the sensor and you get your crazy (additional digital data signals) readings.

I first made a low pass filter to remove those additional signals, but they seemed to be around the same frequency as ~5mph sensor readings.

In the end, I added a hall effect switch that uses the same magnet the tach sensor (reed switch) does. The output from the hall effect sensor runs to an optocoupler on my circuit, then to my board.

If you start the motor running, you can completely remove the sensor and you get your crazy (additional digital data signals) readings.

It strikes me as pretty unlikely that the manufacturers would lie about the way the reed switch works, and also pretty unlikely that they would go to all the bother of trying to piggy-back a digital signal on a circuit that was being mechanically shorted by a reed switch at uncontrolled intervals. It's just not a sensible thing to do, and everything else that has been described seems perfectly sensible and straightforward. More likely IMO, what you were picking up is some combination of electrical noise picked up by the wiring loop and/or a floating input.

I only provide help via the forum - please do not contact me for private consultancy.

The MC-2100 has its own processor and software, which allows it to communicate with the console. This is done by a small digital signal carried by the GREEN tach wire. By entering calibration mode on the console, two alternate screens can be accessed which give information on the controller, including the status of the troubleshooting LED, motor voltage, and motor amperage. This greatly increases the amount of troubleshooting that can be done without removing the treadmill's motor hood.Some MC-2100 controllers will have the transformer mounted on the circuit board as shown above. Other versions..........

The above is an excerpt from the 'datasheet' on page 1. I found it crazy as well, and initially just thought it was motor noise.